Metal-plastic composite made from long-fiber-reinforced thermoplastics

ABSTRACT

The invention relates to metal-plastic composites in which the coefficients of thermal expansion of the plastic structures used are similar to those of the metals used, and whose strengths and stiffnesses are superior to those of purely metallic structures.

[0001] The present invention relates to a component made fromlong-fiber-reinforced thermoplastics and characterized by bondingbetween metal structures and plastic structures. The advance ofautomization in motor vehicle assembly makes it necessary, or at leasthighly desirable, that assemblies of rigid and movable parts can, wherepossible, themselves be put together and tested for correct functioningprior to their actual final installation into the motor vehicle as it isproduced.

[0002] Load-bearing structures used in vehicle construction and inindustrial applications are usually composed of metals. In thisconnection it has been found that a considerable rise in stiffness andin strength can be brought about by using cross-ribbing, for example. Asimilar metal-plastic composite is described in EP 0 370 342 B1.

[0003] It is possible here to reduce the wall thickness for a givenload, and thus to make a considerable saving in weight.

[0004] These load-bearing structures may serve as mounting supports(front end, door module, dashboard support). For this purpose, a highlevel of mechanical properties is required, and this level can beprovided by metal-plastic composites. In addition, thermoplasticallyprocessable plastic gives the high degree of integration which allowslow-cost design.

[0005] However, a disadvantage is that the coefficients of thermalexpansion of the plastics used here differ from those of the metals.During processing, and during use in metal-plastic composites used overa wide temperature range, these differences cause internal stresses anddistortion, which in turn reduce load-bearing capacity and acceleratematerial fatigue. These problems are indicated in lines 15-18, column 2of EP 0 370 342 B1. The presence of the disadvantages described abovecould be deduced from the mention of the fact that the coefficient ofthermal expansion of a metal-plastic composite is essentially determinedby the metal.

[0006] DE 38 18 478 A1 describes a composite material comprising a metallayer and comprising a crosslinked polypropylene layer, fiber-reinforcedwith a glass fiber mat and having a coefficient of thermal expansionsimilar to that of the metal used. A disadvantage here is that injectionmolding is made difficult or impossible by the use of a glass fiber matand by the crosslinking. The crosslinking also makes it impossible torecycle the plastic.

[0007] The object of the invention was to produce metal-plasticcomposites in which the coefficients of thermal expansion of theuncrosslinked plastic structures used are similar to those of the metalsused, and whose strengths and stiffnesses are superior to those ofpurely metallic structures, while their weight is identical or lower.

[0008] Surprisingly, it has now been found that the coefficients ofthermal expansion of long-fiber-reinforced thermoplastics are similar tothose of steel, aluminum, and magnesium, and that thelong-fiber-reinforced thermoplastics have less tendency to creep thanshort-fiber-reinforced thermoplastics. Using these materials it ispossible to produce metal-plastic composites with strengths andstiffnesses superior to those given by purely metallic structures, andwith weights below those of purely metallic structures.

[0009] The invention therefore provides a metal-plastic compositecomprising at least one metal and comprising at least onelong-fiber-reinforced thermoplastic whose coefficient of thermalexpansion is similar to that of the metal used. The termlong-fiber-reinforced thermoplastics is generally used forthermoplastics reinforced with fibers whose length is at least 0.5 mmand not more than 50 mm. The length of the fibers is preferably from 1mm to 25 mm, in particular from 1 mm to 12 mm. The length of the pelletsand the length of the fibers are mostly identical in these materials.The reinforcing fiber is not restricted to a particular material. It ispreferable here to use fibers made from materials with high meltingpoints, for example glass fibers, carbon fibers, metal fibers, oraromatic polyamide fibers. The fibers within the pellets here may havebeen completely impregnated with the thermoplastic, or be in the form ofa glass-fiber bundle coated by plastic.

[0010] Ribs made from material such as polypropylene are injected intometallic structures. Polypropylene was selected as the lowest- densityplastic. The use of long fibers as reinforcing material allowscoefficients of thermal expansion similar to those of metals, and lowtendency to creep, to be achieved without any need to crosslink thethermoplastic used.

[0011] The mechanical properties of the long-fiber-reinforcedthermoplastic are markedly superior to those given byshort-fiber-reinforced thermoplastics.

[0012] Use of long-fiber-reinforced thermoplastics allows dramatic risesin strength and in stiffness to be achieved, together with a lowtendency to creep and a coefficient of thermal expansion similar to thatof metals.

[0013] The loading placed on the metal-plastic bonds in themetal-plastic composites described was so high that here again theadvantages in strength and in stiffness overshort-glass-fiber-reinforced thermoplastics were required in order toproduce a component which could be subjected to high load.

[0014] The component according to the invention may generally becomposed of metal structures of any desired metals, and isadvantageously composed of iron or steel (including high-alloy orstainless), aluminum, magnesium or titanium.

[0015] To improve adhesion, the surface may advantageously have beenprovided with adhesion promoters, primers or surface coatings.

[0016] According to the invention, plastic materials which may be usedare long-glass-fiber-reinforced or carbon-fiber-reinforcedthermoplastics based on polyethylene, polypropylene, polyacetal,polyamide, polyester, polyphenylene oxide, polyphenylene sulfide,polyurethane, polycarbonate or polyester oracrylonitrile-butadiene-styrene copolymers or onacrylonitrile-styrene-acrylate graft polymers, or blends made from theplastics mentioned. Particular polyesters which may be used arepolyethylene terephthalate or polybutylene terephthalate.

[0017] Plastic materials which may be used, besides freshly producedmaterials, are first-, second- or higher-generation recycled materials,or mixtures made from freshly produced material with recycled materials.Mixtures of this type may, if desired, also comprise additives, or mayhave been modified by admixture of other compatible polymers. There isno need for crosslinkers to be added in order to achieve the advantagesof the invention.

[0018] Besides the long reinforcing fibers, the plastic material mayalso comprise other conventional additives and reinforcing materials,for example other fibers, in particular metal fibers, or mineral fibers,processing aids, polymeric lubricants, ultra high-molecular-weightpolyethylene (UHMWPE), polytetrafluoroethylene (PTFE), or graftcopolymer, which is a product from a graft reaction, made from an olefinpolymer and from an acrylonitrile-styrene copolymer, antioxidants,adhesion promoters, nucleating agents, mold-release aids, glass beads,mineral fillers, such as chalk, calcium carbonate, wollastonite, silicondioxide, talc, mica, montmorillonite, organically modified orunmodified, organically modified or unmodified phyllosilicates,materials which form nanocomposites with the plastic, nylonnanocomposites, or mixtures of the substances mentioned.

[0019] The coefficient of thermal expansion is similar to that of ametal if it does not deviate by more than 20×10⁻⁶K⁻¹ from thecoefficient of thermal expansion of the metal used.

[0020] The plastic structures may be produced by thermoplasticprocessing methods, preferably by conventional techniques, such asinjection molding, thermoforming, hot-press molding,injection-compression molding, low-pressure injection molding or blowmolding.

[0021] The cross section of the metal structures preferably has theshape of a U, V or W. Within these metal structures, the shapes of theplastics may be as desired, extending to sheet-like layers, and theplastics may have been provided with functional parts, such as housingsor housing sections, snap connectors or film hinges. Since abrasionperformance with respect to plastic and metal is good, the functionalparts may advantageously be sliding surfaces. These plastic structurespreferably have the shape of ribs.

[0022] According to the invention, there are two different ways ofproducing the metal-plastic bonding. The first method uses one of thethermoplastic processing methods to bring about bonding within the metalstructure.

[0023] The bonding is preferably produced by interlocking, under-cutting (e.g. by using a dovetail shape) or penetration through anaperture or slot, where a plug is produced on the reverse side of theaperture and cannot be pulled back through the aperture without beingdestroyed. In the second possibility, the metal-plastic bonding isbrought about by introducing elevations of peg-like or other shape onthe plastic part into openings in the metal structure, for exampleapertures or slots. A permanent connection is produced advantageously bysubsequent heat welding, bending or thermal deformation.

[0024] The coefficients of thermal expansion of long-glass-fiberreinforced thermoplastics are similar to those of steel, aluminum andmagnesium (Table 1). Material Coefficient of thermal (Plastic-Fiber,Amount of expansion fiber/weight %) 10⁻⁶ K⁻¹ PP-GF 30 16 PP-GF 40 15PP-GF 50 13 PA66-GF 40 19 PA66-GF 50 17 PA6G-GF 60 15 PA66-CF 40 13PET-GF 40 16 PBT-GF 40 19 PPS-GF 50 12 PC/ABS-GF 40 18 TPU-GF 40 13TPU-GF 50 10 TPU-CF 40 18 Comparison with metals Iron   12.2 Steel 12Magnesium 26 Aluminum 22 Comparison with unreinforced plasticsUnreinforced PP 83 Unreinforced PA66 90

[0025] Table. 1: Coefficients of thermal expansion oflong-fiber-reinforced thermoplastics (from −30° C. to +30° C.);PP-polypropylene, PA-polyamide, PET-polyethylene terephthalate,PBT-polybutylene terephthalate, PPS-polyphenylene sulfide,PC/ABS-polycarbonate-ABS-blend, TPU-thermoplastic polyurethaneelastomer, GF-glass fiber, cf-carbon fiber

[0026] Long-fiber-reinforced thermoplastics also have a lower tendencyto creep than short-fiber-reinforced thermoplastics. The invention isfurther illustrated by FIG. 1. FIG. 1 plots the percentage elongationagainst load duration. Curve 1 shows the creep performance of ashort-glass-fiber-reinforced nylon-6,6 with a proportion of 30% of glassfibers, and curves 2 and 3 show the creep performance oflong-glass-fiber-reinforced polypropylene with a proportion of 40% and50% of glass fibers.

[0027] The creep performance and coefficients of thermal expansion oflong-glass-fiber-reinforced thermoplastics make them particularlysuitable for use in metal-plastic composites used over a widetemperature range, as is the case in the automotive industry, forexample (from −40 to +120° C.).

1. A metal-plastic composite comprising at least one metal andcomprising at least one uncrosslinked long-fiber-reinforcedthermoplastic whose coefficient of thermal expansion is similar to thatof the metal used.
 2. The metal-plastic composite as claimed in claim 1, where the plastic used comprises polyethylene, polypropylene,polyamide, polyacetal, polyester, polyphenylene oxide, polyphenylenesulfide, polyurethane, polycarbonate, polyester,acrylonitrile-butadiene-styrene copolymers oracrylonitrile-styrene-acrylate graft polymers, polyethyleneterephthalate or polybutylene terephthalate, or comprising a mixturemade from at least two of these plastics.
 3. The metal-plastic compositeas claimed in claim 1 , where the fibers present in the plastic compriseglass fibers, carbon fibers, metal fibers or aromatic polyamide fibers.4. The metal-plastic composite as claimed in claim 1 , where the lengthof the polymer pellets used for the production process and the length ofthe reinforcing fiber are identical.
 5. The metal-plastic composite asclaimed in claim 1 , obtainable by conventional thermoplastic processingmethods, such as injection molding, thermoforming, hot-press molding,injection-compression molding, low-pressure injection molding or blowmolding.
 6. The metal-plastic composite as claimed in claim 1 , wherethe metal used comprises iron, steel, aluminum, magnesium, or titanium.7. A process for producing a metal-plastic composite as claimed in claim1 , in which the coefficient of thermal expansion of the plastic issimilar to that of the metal used, the plastic used being athermoplastic reinforced with fibers of length from 0.5 mm to 50 mm. 8.The use of the metal-plastic composite as claimed in claim 1 , forproducing load-bearing structures in front ends, in door modules, indashboard supports, or in other mounting supports.